Beyond the Salt Crust: On Combined Evaporation and Subflorescent Salt Precipitation in Porous Media

The impact of salt precipitation, within or on top of porous media, on water evaporation from the media is an important issue with practical implications for agricultural practices, civil engineering and construction materials, the food industry and more. Even though the evaporation of saline solutions from porous media has been widely studied in recent years, there are still many uncertainties about the associated physical and chemical mechanisms. Moreover, most studies have focused on the impact of efflorescent salt precipitation on evaporation. This work focuses on subflorescent salt precipitation and its connection to evaporation from porous media. The different impacts of subflorescent and efflorescent salt precipitation on porous media evaporation are discussed, and the results of experiments that measured and quantified the impact of crystallization patterns on evaporation and porous media hydraulic conductivity are presented. A simple model was used to better understand the limiting factors that control liquid transport within porous media, with and without crystallized salts. The experimental results showed a major impact of efflorescent salt crust on evaporation, while for the subflorescent salt precipitation, the measured evaporation rates were similar to those measured for salt-free conditions. The model indicated that even though subflorescent-precipitated salt reduced the hydraulic conductivity of the medium by several orders of magnitude, it was not the limiting factor for evaporation as the diffusive vapor transport between the matrix surface and atmosphere was the slowest transport mechanism in the system, thus controlling evaporation rates.